PROJECT SUMMARY Effective host defense against viral pathogens requires antigen-receptor activation and cooperating signaling (cosignaling) from cell surface molecules and cytokines. The herpesvirus entry mediator (HVEM; TNFRSF14), a member of the TNF Receptor superfamily, serves as a molecular switch between proinflammatory and inhibitory cosignaling pathways initiated by coreceptors, LIGHT (TNFSF14) and the immunoglobulin superfamily member BTLA (B and T lymphocyte attenuator). New results indicate effective memory T cell differentiation requires the HVEM cosignaling system, however, viral pathogens usurp the HVEM pathway to thwart effective host defense. This project focuses on the HVEM system in viral latency. We recently discovered that Herpes Simplex virus (HSV)-1 requires LIGHT-HVEM-BTLA system to maintain latency in the trigeminal ganglia in a mouse ocular infection model. Our preliminary evidence indicates that HSV-1 is unable to efficiently maintain latency in mice genetically deficient in HVEM, BTLA or LIGHT and the viral latency- associated transcript (LAT) uniquely upregulates HVEM expression in latently infected ganglia. Moreover, effector T cells fail to accumulate in the ganglia during latent infection in mice lacking HVEM or HSV deficient in LAT. These results implicate multiple roles for the HVEM cosignaling pathway in HSV-1 infected neurons and in effector T cells controlling latency. We have recruited key collaborators and developed animal and tissue culture models that can be probed using genetic and biochemical approaches to investigate the HVEM cosignaling pathway in viral latency. To accomplish this goal we propose two specific aims: 1) characterize molecular interactions regulating the expression and cosignaling actions of HVEM and its ligands in neuronal and T cell lines, and 2) investigate the requirement of HVEM signaling in vivo in neuronal and lymphoid compartments in latency. This investigation will provide new insight into the mechanisms of the HVEM cosignaling system in regulating viral pathogenesis.